This invention relates to a process for incorporating fibrous filler into styrene-butadiene rubber.
Fibrous fillers have been incorporated into plastics and elastomers for the purpose of providing additional strength to articles fabricated from the polymers, obtaining good surface contact properties for articles such as power transmission belts, and reducing compound cost by serving as low cost fillers. Fibrous fillers have been added to plastics and elastomers by heating the polymers to soften them and thoroughly mixing the polymer and filler on a mill or in an internal mixer. This procedure has inherent drawbacks when fibers are incorporated in certain elastomers. The need for incorporating fibers into elastomers is critical for many uses of articles fabricated from elastomers such as, for example, power transmission belts, tires, etc. The procedure now used on a commercial scale by the fabricator is to mix the solid uncured elastomer with the fibrous filler in a BANBURY mixer or on a rubber mill. Mixing is continued for about 5 to 10 minutes. After that time mixing must be discontinued for a substantial amount of time because the elastomer becomes overheated, which, if mixing is continued, would degrade the elastomer and result in substantial lowering of the important properties of the elastomer and/or scorching of the stock. When the mixture of the elastomer and fiber overheats, it must be cooled before mixing is continued. This procedure of mixing to incorporate the filler in the elastomer and cooling due to heat build-up may require as many as six sequences. Moreover, progressive working of the rubber can produce an unusable scorched product before an adequate mix is even possible, especially with aramid fibers in commercial scale mixers when cooling capacity is limited. The incorporation of the fibrous fillers into the elastomer by prior art methods is both energy intensive and expensive due to the long times required by the fabricator to incorporate fiber into the elastomer. The present invention provides a process for incorporating organic fibrous fillers into a styrene-butadiene rubber which is economical, readily accomplished and minimizes the dispersive work necessary to achieve a given compound quality.